1. Field of the Invention
This invention relates to a shaped ceramic composite article comprising ceramic oxide fiber(s), a first coating comprising a carbonaceous matrix which includes boron nitride particles in contact therewith, and a second coating comprising silicon carbide. In another aspect, the present invention provides a method of making the same.
2. Description of the Related Art
Radiant burner tubes are used in high temperature, corrosive environments such as that found in industrial heat treating furnaces and aluminum melting furnaces. The three most common types of commercially available radiant burner tubes are metal alloy (e.g., nickel-based superalloy) tubes, ceramic (e.g., silicon carbide) monolith tubes, and ceramic composite (e.g., ceramic fibers and ceramic cloth in a ceramic matrix) tubes. The upper use temperature of such tubes is typically in the range from about 900.degree. (1650.degree. F.) to about 1260.degree. C. (2300.degree. F.).
Although monolithic silicon carbide radiant burner tubes with an upper use temperature up to about 1260.degree. C. are available, such tubes are typically very brittle and prone to fail, a common problem of conventional, shaped ceramic monoliths.
While it is possible to select a ceramic composite from which to prepare a radiant burner tube which meets most, but not necessarily all, of the requirements for use in high temperature, chemically corrosive environments, it is only possible by taking great care in the selection and by making some compromises in properties.
A commercially available ceramic composite radiant burner tube is marketed, for example, under the trade designation "SICONEX" by the Minnesota Mining and Manufacturing Company (3M) of St. Paul, Minn. "SICONEX" radiant burner tubes are a ceramic-ceramic composite comprised of aluminoborosilicate ceramic fibers, a carbonaceous layer, and a silicon carbide layer. "SICONEX" radiant burner tubes are prepared by braiding, weaving, or filament winding aluminoborosilicate ceramic fibers in the shape of a tube, or alternatively, fashioning aluminoborosilicate ceramic cloth into a tube shape. The ceramic fiber tube shape is treated with a phenolic resin which is cured, producing a rigidified article. The rigidified article is heated in an evacuated chamber such that the cured phenolic resin is carbonized. The article is then coated with silicon carbide via chemical vapor deposition at temperatures ranging from about 900.degree. to about 1200.degree. C. to provide a semi-permeable, chemically resistant coating of silicon carbide. The resultant rigid ceramic composite tube is useful at high temperatures in corrosive environments.
The upper use temperature of "SICONEX" radiant burner tubes under typical operating conditions is about 1260.degree. C. Above about 1260.degree. C. such tubes typically exhibit properties characteristic of ceramic monoliths (i.e., brittleness). There is a long-standing need to improve the upper temperature limit and the mechanical characteristics of such a composite.
While there have been many approaches to improving mechanical characteristics of ceramic composites, such efforts have rarely been coupled with a significant improvement in the high temperature performance of the composite.
For example, U.S. Pat. No. 3,672,936 (Ehrenreich) discloses a reinforced carbon article which comprises a carbon fiber shape bonded by a carbon binder and having incorporated within the article the in situ reaction product of carbon and a boron-containing additive which comprises a material selected from the group consisting of boron, boron nitride, boron silicide, and refractory metal borides. The reinforced carbon article is made by forming a carbon fiber shape, dispersing the boron-containing additive within at least a portion of the carbon fiber shape, impregnating the carbon fiber shape with a carbonizable binder, and heating the shaped article to carbonize the binder and to form in situ the reaction product of carbon and the boron-containing additive.
U.S. Pat. No. 3,565,683 (Morelock) teaches a method of depositing a boro-carbon coating onto filaments, wherein an electrically heated surface of a pyrolytic carbon coated fused silica or quartz filament is passed through a liquid, thermally decomposable boron compound such as boron trichloride dissolved in a non-polar organic solvent such as benzene. The heated portion of the filament produces an envelope of solvent vapor and boron trichloride gas which are pyrolytically decomposed and carbon and boron are simultaneously deposited on the fiber.
U.S. Pat. No. 4,605,588 (Simpson et al.) discloses a process for creating a substantially uniform boron nitride barrier coating on the surface of oxide-based ceramic fibers, wherein an oxide-based ceramic fiber containing boron is heated for about 5-90 minutes in a nitriding atmosphere of ammonia, hydrogen and nitrogen at a temperature of between about 2200.degree.-2600.degree. F. to diffuse boron from the fiber to the surface or slightly within the fiber where it reacts to form the boron nitride coating.
U.S. Pat. No. 4,642,271 (Rice) discloses a ceramic fiber composite material comprised of boron nitride coated ceramic fibers (e.g., SiC fibers, Al.sub.2 O.sub.3 fibers, and graphite fibers) embedded in a ceramic matrix (e.g., SiC, ZrO.sub.2, 96% SiO.sub.2 with 4% B.sub.2 O.sub.3, mullite, cordierite, and carbon).
U.S. Pat. No. 4,650,775 (Hill) describes a thermally-bonded fibrous product composed of a sintered blend of aluminosilicate fibers, silica powder, and boron nitride powder.
U.S. Pat. No. 4,751,205 (Hill et al.) teaches a thermally-bonded fibrous product composed of a sintered blend of ceramic fibers, low-grade silica material, and boron nitride.
U.S. Pat. No. 4,752,503 (Thebault) discloses a thin, refractory, intermediate adhesive layer of laminar structure (e.g., pyrocarbon or boron nitride) deposited in an oriented fashion by chemical vapor deposition onto reinforcing fibers, wherein the intermediate layer has a greater elongation at break than the matrix and has a thickness of between 0.2 and 3 micrometers.
U.S. Pat. No. 4,766,013 (Warren) describes a fibrous ceramic matrix composite article said to be useful in corrosive environments. The composite article comprises a porous carbon fibrous substrate or other suitable high temperature fibrous substrate which may include a pyrolytic carbon or appropriate chemical vapor deposited sheath formed about each fiber of the substrate; a chemically vapor deposited metallic carbide, oxide, boride or nitride coating over the coated fibers of the substrate; and an impermeable metallic carbide, oxide, boride, or nitride outer protective layer formed about the entire periphery of the coated substrate.
U.S. Pat. No. 4,970,095 (Bolt et al.) teaches an improved method for depositing boron nitride coatings on ceramic fibers.
U.S. Pat. No. 4,981,822 (Singh et al.) discloses a composite article produced by depositing a slurry of infiltration-promoting material and organic binding material on a layer of boron nitride-coated fibrous material forming a tape therewith on drying, firing the tape to burn out organic binding material, and infiltrating the resulting porous body with a molten solution of boron and silicon. Patentees state that in carrying out the inventive process, the boron nitride is to be coated on the fibrous material to produce a coating thereon which leaves no significant portion, and preferably none, of the fibrous material exposed.